1. Field of the Invention
The invention relates to apparatus, and an accompanying method for use therein, for a dynamic material testing system that has independent control over specimen deformation and strain rate. In addition to controllably deforming a work zone of a specimen, the system can also simultaneously direct resistance heat or conductively cool the specimen, under controlled conditions, in order to establish isothermal planes at a desired substantially uniform temperature throughout the work zone.
2. Description of the Prior Art
Metallic materials play an indispensable role as an essential component of an enormous number of different products. Currently, these materials are typically fabricated through rolling, forging or extruding operations into sheet, strip or wire, i.e. intermediate products, which are thereafter appropriately formed into a shape of a final product. Unfortunately, the financial costs associated with establishing a production mill to produce an intermediate product are staggering.
Nevertheless, an increasing number of new rolling mills has come on-line within the last few years. As a result, mill operators are experiencing very intense marketplace competition for finished rolled stock which, in turn, necessitates that mills be run to produce rolled material of constantly improving quality at ever decreasing cost. Not unexpectedly, this competition is becoming increasingly more intense as additional production facilities come on-line. Accordingly, in view of this competition and the enormous cost associated with establishing and operating a rolling mill, profit margins on rolled stock tend to be rather slim. As such, marketplace economics now dictate that a mill must be established and operated with as little margin of error as possible. Consequently, to reduce various financial risks to manageable levels, a substantial need has arisen to accurately simulate rolling operations in, typically, a laboratory environment and learn their effects on material characteristics before implementing these operations on an operating production mill. The simple reason for this is that once a mill is in production, any subsequent down-time tends to be extremely costly and thus is best avoided. Inasmuch as accurate simulations permit mill parameters to be optimized off-line, then, by substantially eliminating mill down-time, such simulations advantageously permit higher production yields while significantly reducing research costs. Fortunately, simulations are substantially less capital intensive, typically by at least several orders of magnitude, and significantly less time-consuming than establishing a so-called small pilot mill or, where possible, even a small sample mill in order to obtain the desired metallurgical results. Furthermore, since rolling mill simulations can yield extensive data from specimens that are considerably smaller than normal production sized materials, appreciable material savings and hence significant cost economies result if a production mill were to be accurately simulated rather than being taken off-line (out of service) to provide test materials. Accordingly, a substantial and growing need exists in the art for apparatus that can physically simulate a rolling mill with a high degree of accuracy.
Ideally, a proper rolling mill simulation should permit a speciment to undergo the same mechanical deformation and thermal processing that will be encountered in a desired rolling mill. While an accurate simulation is relatively easy to perform for a rolling mill having a single stand, accurate simulations become increasingly difficult to accomplish for multi-stand mills. Specifically, with modern rolling mills, strip stock is often passed through a multi-stand mill at a relatively high velocity. These mills quickly deform the material using several deformations in series and obtained through successive roll stands within a relatively short time, often on the order of a few milliseconds, between successive deformations. In addition, the temperature of the strip also tends to change from each stand to the next.
Several techniques exist in the art for simulating a rolling mill. One well-known technique, as described in, for example, U.S. Pat. Nos. 4,109,516 (issued to J. Fuxa on Aug. 29, 1978) and 3,457,779 (issued to E. Hahn et al on July 29, 1969), involves use of the so-called cam plastometer. Here, a specimen is fixedly held. A ram controllably and rapidly strikes the specimen with a given stroke distance to deform the specimen in an amount equal to that which would be encountered in a mill stand. The motion of the ram is governed by a cam follower that rotates against a cam, with the latter being rotated by an hydraulic motor and a flywheel. Each rotation of the cam moves the ram to compress the specimen once. The size and shape (profile) of the cam, particularly that of its lobe, is set to obtain desired amounts of true strain and true strain rate in the deformed specimen. Unfortunately, this technique possesses several drawbacks which limit its utility. First, cam plastometers are unable to simulate modern multi-stand mills. In this regard, older cam plastometers have generally been limited to imparting a single amount of deformation to a specimen and thus could only simulate a single rolling mill stand. Recently, cam plastometers have been developed that are capable of imparting two successive deformations to a specimen thereby possessing a capability to simulate two stand mills. However, modern rolling mills frequently possess more than two stands and frequently four or more. Cam plastometers have not yet been developed that can simulate that many mill stands. Furthermore, while a cam plastometer can accurately reproduce a strain profile for a rolling operation, a cam must be replaced by one having a significantly different cam profile if a widely differing strain profile is to be generated by that plastometer. The operations inherent in machining a new cam with an appropriate profile and then substituting one cam for another each time a different strain profile is desired are both time consuming and inconvenient. Consequently, cam plastometers, due to their limited simulation ability and attendant difficulties of use, are presently being employed less and less for mill simulation.
A second well-known technique for simulating a rolling mill generally involves using a computer-controlled material testing system in which a specimen is grasped in a fixed mount and then controllably struck by a ram mounted, via a rod, to an end of an servo-controlled hydraulic piston. The rate of travel of the ram determines the strain rate of the specimen with the distance through which the ram compresses the specimen determining its deformation. Unfortunately, this technique generally suffers from excessive dwell time, poor end-of-stroke control and deviations from a programmed true strain rate during deformation, all of which adversely limit the ability of this technique to accurately simulate modern medium to high speed multi-stand rolling mills.
Specifically, to simulate each stand of a multi-stand rolling mill with this second technique, the computer situated within the material testing system would be appropriately programmed by an operator to provide a controlled series of "hits" to the specimen, with each hit causing a desired strain rate and compressive deformation in the specimen. Once a simulation commenced, the computer would first set the speed and stroke distance of the ram to provide the desired strain rate and deformation for the first hit on the specimen, then control the hydraulic system to retract the ram into its starting position and thereafter appropriately operate a hydraulic servo-value that controls the piston to permit the ram to strike the specimen with the proper velocity for this hit and then continue compressing the specimen for a desired distance, with this process repeating for each successive hit.
Mechanical reality, as dictated by the laws of physics, is such that as the speed of the ram increases, higher forces are needed to stop and reverse its direction. In a modern medium to high speed multi-stand rolling mill, the typical material transit time between the last and next-to-last roll stands therein is on the order of tens of milliseconds and sometimes as small as only a few milliseconds. Furthermore, rapid deformation of the type encountered in a rolling mill necessitates that the ram travel at a high rate of speed when impacting the specimen for each hit. Accordingly, if the performance of such a mill is to be accurately simulated, then the dwell time between the next-to-last and last deformations that are imparted to a specimen should match this transit time. Unfortunately, to provide such a short dwell time, unmanageably large forces would likely be required to: (a) stop the ram at the end of the next-to-last hit, (b) then reverse its motion to fully retract it into its starting position, and (c) finally strike the specimen at high speed to cause the desired strain rate in the specimen for the last hit--all within a comparable time interval. Furthermore, apart from the necessary forces, hydraulic servo-values and associated hydraulic components often do not provide a sufficiently fast response to accommodate the required movement of the ram. This, in turn, also tends to increase the minimum dwell time of the system and thereby reduce the maximum rate at which the specimen can be successively hit.
Furthermore, control of the position and velocity of the ram at the end of its stroke tends to be quite imprecise. Specifically, during simulation of multi-stand mills, the height of the specimen incrementally decreases as it is successively hit. Inasmuch as true strain, .epsilon., for compressive deformation is defined by: .epsilon.=-ln(h.sub.0 /h), where h is the final specimen height and h.sub.0 is the initial height, the true strain rate, (d.epsilon./dt), is given by: (1/h) (dh/dt). Thus, if the final thickness of the specimen is relatively small, then the true strain rate obtained during the final hit may be correspondingly high. For example, if a ram moving at a controlled stroke rate of 1000 mm/second hits the specimen to yield a final compressive deformation that reduces the specimen to 2 mm in height, then the initial true strain rate at impact in the specimen, i.e. the "entrance" true strain rate, is 500/second.
Relatively low strain rates, e.g. 2/second, such as that typically encountered in rolling plate in a single stand mill, and corresponding final deformations in a test specimen can be readily obtained using conventional servo-hydraulically actuated rams. However, relatively high strain rates, such as illustratively 100-500/second or even less as would typically be encountered in the final stand in a medium to high speed rolling mill, are very difficult to accurately obtain in a test specimen using such a ram. At such high strain rates, the ram must travel and accelerate over a given distance in order to impact the specimen at a velocity that will produce a correct entrance true strain rate in the specimen. However, in practice, the ram generally overshoots and does not stop at the exact position at which a desired amount of deformation is produced in the specimen. For small deformations, the ram velocity can not be maintained without significant over-travel of the ram. This over-travel produces unwanted additional strain in the specimen. As the strain rate increases and the specimen deformation accordingly decreases for successive hits, the additional strain caused by over-travel becomes relatively large compared to the initial specimen height prior to a hit. Since the final stand(s) of a modern multi-stand rolling mill typically imparts a relatively small deformation but a relatively large true strain rate to rolled strip, the strain in the specimen, including that produced by over-travel of the servo-hydraulically actuated ram, by a conventional material testing system significantly deviates from the strain which would be produced by each of these stands in the mill thereby corrupting the simulated results. If the over-travel is relatively significant as compared to the desired specimen height, then the additional strain imparted to the specimen becomes intolerably high. Concomitantly, if the desired deformation is achieved at the instant the ram stops moving, then the ram velocity and hence the resulting true strain rate are both generally too low, particularly near the end of its movement, to accurately simulate actual mill conditions in the specimen.
As to programmed true strain rate, if a conventional servo-hydraulically actuated ram produces a correct programmed entrance true strain rate, then the precision with which the servo in the material testing system will maintain this rate throughout the ensuing deformation will typically be based on a ratio between the time during which the speimen is deformed, i.e. the deformation time, and the response time of the system. As either the response time shortens or deformation time increases, then increasingly precise true strain rate control can result. Hence, the ram would be increasingly able to produce a desired true strain rate at any position along its stroke during the deformation. However, since response time is often comparable to a fast deformation time, a servo-hydraulically actuated ram is generally incapable of accurately changing the true strain rate it imparts to a specimen during a fast deformation. For that reason, relatively precise control of the true strain rate produced during a fast deformation can only be accomplished if the material testing system is operated such that the ram moves at a correct velocity upon impacting the specimen and does not stop as soon as the desired strain is attained but rather continues to move thereafter. Unfortunately, this causes over-shoot. As discussed above, the additional strain caused by ram over-shoot, particularly at high true strain rates and small deformations, can produce gross inaccuracies in simulating actual conditions associated with the final stand(s) of a medium to high speed multi-stand rolling mill.
Consequently as the speed of modern multi-stand rolling mills increases, the inability of conventional material testing systems that employ servo-hydraulically actuated rams to provide the requisite high speed control over strain rate and deformation causes these systems to become increasingly less suitable for simulating these mills.
In an attempt to alleviate these limitations, it is known in the art to place an adjustable height mechanical stop between a moving platen to which the ram is mounted and a fixed platen to which the specimen is secured. This stop may comprise one or two wedges. The base of each wedge abuts against the fixed platen with its inclined surface facing the moving platen and particularly a complementary shaped surface thereon. The wedge(s) is connected to a servo-hydraulic cylinder and can be extended or retracted thereby in a direction transverse to ram displacement to provide a stop of a desired thickness between the two platens. In operation, the wedge(s) is first positioned such that the ram produces a desired amount of deformation in the specimen. Thereafter, the moving platen is accelerated towards the fixed platen with movement of the former being halted upon contact with the wedge(s). The area of each wedge is much larger than that of the cross-section of the specimen so that a relatively small amount of elastic strain is produced in each wedge when it is struck by the moving platen. While this arrangement advantageously eliminates over-travel of the ram and its attendant adverse consequences, it disadvantageously requires that, for a constant true strain rate, the ram be decelerated during each hit and that the ram stop at a new position at the end of each successive hit. This, in turn, greatly complicates the programming of such a system in order to achieve a true mill process simulation. Specifically, when programs for use in such a material testing system are written to control the movement of the ram, the program must cause the ram to decelerate while the specimen is being deformed if the true strain rate is to be either held constant or controlled in accordance with the performance of the mill. Since the ram is traveling at a high velocity at the beginning of the hit and the total amount of deformation during a hit may be very small (the ram travel may illustratively be a fraction of a millimeter for some "final" hits), then, owing to response limitations of the system, properly slowing (decelerating) the ram to produce a constant true strain rate (or other programmed rate) in the specimen may be quite difficult to achieve in practice. Furthermore, the changing stop position of the ram for each successive hit further complicates programming the needed ram deceleration and hence significantly increases the difficulty associated with programming a true mull process simulation.
Thus, a need still exists in the art for a material testing system, and specifically for apparatus for inclusion therein, that can accurately hit a specimen under test to produce a desired amount of deformation therein at a relatively high true strain rate but without generating substantially any over-travel. In addition, this apparatus should permit a desired relatively high entrance true strain rate to be produced in the specimen and then maintained throughout the ensuing deformation. Both the true strain rate and the deformation should be independently adjustable. Furthermore, the apparatus should be capable of successively generating such hits with reduced dwell time. Such a system would advantageously find use in accurately simulating modern high speed multi-stand rolling mills.